Technical Field
The present disclosure is directed to an apparatus and method of treatment of contaminant emissions from carbon capture facilities and, in particular, to an improved apparatus and method of simultaneously controlling and/or treating emissions of amines, nitrosamines, nitramines and aldehydes from flue gases in a cost efficient and/or effective manner.
Background Art
In general, contaminant emissions from carbon capture facilities are of great concern in the industry due to their potential hazardous effect on the environment and/or humans. There are currently no standard carbon capture exhaust gas treatments for, e.g., nitrosamines, nitramines, and the like, and those for amines generally rely on, for example, washwater scrubbers employing pH adjustment of washwater and/or higher washwater replacement rates. The concentrations of contaminants in washwater provide one of the best estimates of contaminant emissions from carbon capture facilities to downwind communities. Systems generally configured as scrubbers designed to remove contaminants from the flue gas currently exist. However, contaminants typically accumulate in the washwater, thus reducing the driving force for mass transfer from the flue gas to the washwater. Additionally, for contaminants that form in the washwater, the mass transfer driving force can work in the opposite direction, such that the washwater would serve as a source of contaminants to the exhaust gas as it is recirculated through the system.
Contaminants of particular concern that generally accumulate in washwater include, e.g., nitrosamines, nitramines, amines, aldehydes, nitrite, and the like. Nitrosamines, nitramines and aldehydes are generally of specific concern due to their potential direct toxicity. Amines are of concern due to their potential to form toxic products downwind. In addition, even at an alkaline pH, aldehydes can catalyze the nitrosation of amines by nitrite. Accordingly, the accumulation of these products in the washwater may foster nitrosamine formation, such that the washwater serves as a source of nitrosamines to the exhaust gas.
Another important N-nitrosamine formation pathway involves nitrite, i.e., a nitrosation pathway relevant to washwaters. Under acidic conditions, formation of the nitrosating agent, e.g., N2O3, generally occurs in a reaction that is second order in nitrite as shown below in Equation 1 (see, e.g., Mirvish, S. S., Toxicology and Applied Pharmacology, 31, p. 325-351 (1975)).2NO2−+3H+→N2O3+H2O+H+  (1)
However, aldehydes have been found to catalyze nitrosation by nitrite, thereby enabling significant formation at neutral and alkaline pH (see, e.g., Keefer, L. K. et al., Science, 181, p. 1245-1247 (1973)). For example, as shown in Equation 2 below, a reaction between dimethylamine and formaldehyde forms a carbinolamine (see, e.g., Casado, J. et al., Journal of the Chemical Society-Perkin Transaction 2, 12, p. 1963-1966 (1984)). With reference to Equation 3, protonation of the carbinolamine, followed by dehydration, generally forms an iminium ion. The iminium ion further generally reacts with nitrite to form a nitrosamine, as illustrated by Equation 4, thereby releasing formaldehyde for further reactions.(CH3)2NH+CH2O→(CH3)2NCH2OH  (2)(CH3)2NCH2OH+H+→(CH3)2N+NCH2OH→(CH3)2N+═CH2+H2O  (3)(CH3)2N+═CH2+NO2−→(CH3)2N—N═O+CH2O  (4)
Due to the high concentrations of aldehydes anticipated to be present in the absorbent solution and/or in the washwater, and because nitrite generally occurs in washwater, the nitrosation pathway is likely to be a significant pathway for nitrosamine formation.
Turning now to FIG. 1, modeled nitrosation rates (M/s) of about 100 μM morpholine by about 400 μM nitrite in the absence (line “a”) and/or presence (line “b”) of about 100 μM formaldehyde as a function of pH are presented (see, e.g., Mirvish, S. S., Toxicology and Applied Pharmacology, 31, p. 325-351 (1975)). Nitrosation of alkylamines by acid-catalyzed nitrosation in the absence of aldehydes is generally maximized at a pH of about 3 to 4. Catalysis of nitrosation by formaldehyde further generally increased nitrosation rates by about four orders of magnitude near a pH of about 10, typical of washwaters.
There are currently two options being pursued in the industry to increase the driving force for transfer of amines from the exhaust gas to the washwater. The first option generally involves the addition of sequential washwater sections. In particular, increasing the number of sequential washwater sections is generally equivalent to increasing the number of transfer units in a plug flow reactor, thereby increasing the removal efficiency of amines. In addition, since nitrosamines and nitramines are typically water soluble, partial removal of nitrosamines and nitramines should also be expected. However, the cost of adding sequential washwater sections is substantially high. Further, these improvements may be temporary, as the accumulation of amines in the washwater generally continues to reduce the mass transfer driving force and reactions of residual NOx with amines in the washwater to form nitrosamines and/or nitramines would cause the washwater to serve as a source of nitrosamines to the exhaust gas.
The second option currently being pursued in the industry generally involves reducing the pH in a single washwater section. In particular, without pH adjustment, most washwaters generally achieve a pH of about 10.5, which is above the dissociation constant pKa (hereinafter “pKa”) of most amines. Reducing the pH below the pKa of amines can significantly reduce their volatility, as the charged forms of amines are non-volatile. Although amines can still accumulate to reach Henry's law equilibrium with the flue gas, a substantial mass of amines could be stored in the washwater in their charged forms. However, upon reaching Henry's law equilibrium, the washwater generally no longer serves as an amine sink. Further, the pKa values for nitrosamines are significantly lower than for amines. Therefore, only at the lowest pH values will acidification aid nitrosamine removal. Additionally, the low pH environment generally promotes the nitrosation of amines by nitrite, thereby permitting the washwater to become a source of nitrosamines.
Both of these options generally rely on phase transfer to reduce losses by storing compounds in the washwater. Therefore, in both cases, the accumulation of the products over time generally reduces the driving force for contaminant removal. Additionally, disposal of washwaters containing these accumulated contaminants is problematic, as the washwaters may be considered hazardous waste.
Thus, a need exists for improved apparatuses and methods of treatment of contaminant emissions from carbon capture facilities. In particular, a need exists for improved apparatuses and methods of simultaneously controlling and/or treating emissions of amines, aldehydes, nitrosamines and nitramines from flue gases in a cost efficient and/or effective manner, and for destroying these contaminants to facilitate the safe disposal of the used washwaters.
These and other needs are addressed by the exemplary apparatuses and methods of the present disclosure.